The most serious problem encountered in the thermal cracking, in a reactor, of a heavy hydrocarbon such as asphalt, coal tar, heavy oil, crude oil, etc., is how to prevent coking from taking place on the interior wall surface of the reactor. Such coking or coke deposition is rather limited in the case of light hydrocarbon such as gaseous hydrocarbons, naphtha, etc., but a stronger tendency toward coking is seen when the hydrocarbon material to be thermally cracked is of the heavy type such as above-mentioned. Therefore, in practicing an industrial cracking of the heavy hydrocarbon, prevention of such coking is a major factor for successful operation of the reactor.
As a measure against such coking and coke deposition, there has been proposed a so-called "wet wall" forming method which is generally employed in the thermal cracking of naphtha. According to this method, a light oil is allowed to flow down into the reactor along its internal wall after the mode of an overflow unit, or a light oil is similarly allowed to flow down from a slit or slits provided at the top of the reactor, so as to form a socalled liquid curtain over the internal wall surface of the reactor. This method, however, has a disadvantage that the reaction product (pitch) might be diluted by the light oil, thereby forming a slurry, in the course of thermal cracking of the heavy hydrocarbon. Even if a portion of the bottom material produced in the course of cracking reaction of the heavy hydrocarbon material in the reactor is taken out of the reactor and the portion of the bottom material is used in place of the light oil for avoiding dilution of the reaction product (pitch), coking can still take place in the sump when the sump is provided at the top. When a slit or slits are provided at the top of the reactor, such slit or slits might be closed by coking, hindering the smooth continuous operation of the reactor.